- In order to explore Shoal Cave, you must first find Shoal Cave. It is the dominant feature of Route 125, a short trip northeast of Mossdeep City. The Cave is smack in the middle of the Route, located beside a pair of trainers. You'll need Surf to reach it.

- The first major thing to note about Shoal Cave is the nature of the water inside. The tide will ebb and rise depending on what time of day you visit Shoal Cave. If you enter during the day at night, or from 9 am to 2 pm, you'll find the waters at their highest. We'll start with high tide.

- In the first area of Shoal Cave you'll find an old man, standing on the edge of the water inside. He'll ask you to bring him some Shoal Salt and Shoal Shells. Bring him four of each and he'll make you a Shell Bell. No problem, old dude.

- Hop into the water and Surf to the bottom-right corner of the screen. You'll find a thin path leading to a dead end. You'll find a hidden Star Piece at this dead end. Hop back into the water and Surf north, where you'll find another room waiting.

- Surf north in the next room. There are small platforms on your right and left as you sail along; climb onto both to find Shoal Shells. Hop back in the water and Surf east to find shallows; there's a Shoal Shell to the north, a Shoal Shell to the south, and a Rare Candy to the west. Head through the exit in the southeast and you'll be back in the first room, not far from a Big Pearl.

- That's all for the current Shoal Cave. Leave and come back either in the early morning (4 am to 9 am) or early evening (3 pm to 7 pm). The layout will be... different.

May searches the Shoal Cave for Shoal Shells and Shoal Salts.

Shoal Cave - Nighttime
- Head into the Cave. (Bring the Mach Bike, as well.) The water has all but disappeared, and when you head north you'll gain access to a cave on the lower floor. This will take you to a set of stairs and a doorway. Go through the door.

- There's a ramp on your right. Use the Mach Bike to get up it. You'll find TM79, Frost Breath, at the top. Go down the ramp on the right side of the TM and you can go up and west to find a pushable boulder; lugging it out of your way will allow you to get at a heap of Shoal Salt. A Martial Artist standing near the Shoal Salt will give you a Focus Band.

- Continue west. There are two ladders. The first from the left leads to a long, roundabout path which will eventually take you to a Shoal Salt and an Ice Heal. It also loops back to the stairs / doorway combo. The second ladder will take you up to two Shoal Salts.

- Return to the room with the Bike ramps, cross them again, and go south. You'll find a ladder leading to an icy area. Slide your way to the south, until you see a semi-puzzle consisting of sliding across an icy obstacle course. Edge just slightly lower than the larger patch of bare ground in the ice, then slide west. If you do this correctly you'll slide past the patch and hit a smaller one to the west. This is just south of a gap in the north that you can fit through to escape the puzzle.

- Head north. Up a small set of stairs you'll find a northern path and some more stairs on your right. To access the stairs, slide up to the bare patch to its left, then slide left, then down, then over to the right. These stairs lead up to an icy rock where you can evolve ice-type pokemon (namely, Eevee into Glaceon), as well as a Never-Melt Ice. Check the north end of this chunk of ground to find Glalitite. Look carefully, as it's difficult to see in this bright area.

- Head north to find another ice patch. Slide north to the far wall, then right to hit an icicle. This will allow you to slide south to a side patch of ground. Hunt around on it (right beside where you land, in fact) to find a hidden Ice Heal.

- Slide north from this patch of land to the nearest, small square of bare ground. Slide east from here, then south, and you'll wind up beside TM07, Hail. Sweet. (Be careful not to jump off the nearby ledge. I did that twice.) That's it for this area.

- Return to the old man. Hand over four of each item listed above and you'll get your Shell Bell. You can return to the Shoal Cave each day, find the ingredients in the same spots, and bring them to the old man for another Shell Bell. The first time you speak to him and complete a Shell Bell, he'll also hand over Slowbronite. Nice.

It feels a bit bigger to me. I don't remember the lower levels being quite so big, and I honestly don't remember the sliding puzzle. I'm pretty sure I only went through Shoal Cave once or twice in the old days, though, so the differences escape me.